The present invention relates to a solder ball that is excellent in micro-adhesion preventing properties and wetting properties and is used for, for example, the connection of electronic parts; and a method for preventing the micro-adhesion of solder balls.
At present, electronic parts obtained by mounting a semiconductor or the like on a substrate are produced by the use of a package having a form called BGA (ball grid array) or CSP (chip size package). In these electronic parts, solder balls are used for bonding of terminals. In general, the bonding of the terminals by the use of solder balls is often conducted in the case of bonding at intervals of hundreds microns and hence is required to have a high dimensional accuracy. In addition, if unsatisfactory soldering occurs in such bonding of the terminals, the package, which is very expensive as compared with the soldering cost, becomes defective. Therefore, the bonding is required to have a high reliability.
For attaining high dimensional accuracy and reliability in the bonding of the terminals, it is important to control the composition of solder balls, soldering conditions and the surface profile of the solder balls. As to the surface profile of the solder balls among them, the degree of oxidation of the surface has to be low. In producing the package by the use of solder balls, the solder balls are mounted at predetermined terminal positions on a BGA substrate and then melted with a heater to form bumps. In this case, if the surfaces of the solder balls are in an oxidized state, the wetting properties of the solder balls at the time of melting are deteriorated, so that the solder does not completely adhere to the terminals on the substrate, resulting in an insufficient bond strength. If a film of dirt, an organic substance or the like is formed on the surface of each solder ball, the wetting properties of the solder balls are deteriorated as in the case of an oxide film. The insufficient bond strength of the solder results in unsatisfactory soldering and hence a defective package.
The oxidation of the surfaces of the solder balls is called blackening because the oxidized surfaces of the solder balls look black. As to the problem of the decrease in bond strength of the solder caused by blackening, there are investigated various methods for suppressing the oxidation of the surfaces of the solder balls by coating the surfaces with a material different from a material for the solder balls. The employment of the following materials as the material for coating the solder balls is investigated: lubricants such as aliphatic hydrocarbon lubricants, higher aliphatic alcohol•higher fatty acid lubricants, fatty amide lubricants, metal soap lubricants, fatty ester lubricants, and the like (for example, JP-A-2000-288771 (patent document 1)); and rust preventives such as metals (e.g. Au and Sn) (for example, JP-A-08-164496 (patent document 2)).
The coating with any of the above-exemplified lubricants or Au suppresses the oxidation of the surfaces of the solder balls. Its concrete action is as follows. The lubricants have an effect of inhibiting the surfaces of the solder balls from becoming chemically active owing to wearing-away of the surfaces by their friction with the outside. In the case of the coating with Au, the oxidation of the surfaces of the solder balls is suppressed by coating the solder surfaces with Au that is not easily oxidizable.
On the other hand, in the case of the coating with Sn, the wetting properties of the solder balls at the time of melting are improved by coating the solder balls with the metal having high wetting properties at the time of melting for a material for the terminals, instead of suppressing the oxidation.
In recent years, as a cause for unsatisfactory bonding in the package, the problem of micro-adhesion of solder balls has been pointed out in addition to blackening. The micro-adhesion of solder balls is a phenomenon remarkably observed in the case of fine solder balls with a diameter of particularly 400 μm or less, more particularly 300 μm or less, and is a state in which originally separated solder balls are adhered to one another by a weak adhesive force. As described above, solder balls have to be mounted on a substrate so that each of them may be mounted at a predetermined terminal position. The mounting of the solder balls is conducted with an automated apparatus. In this case, if there are solder balls which have undergone micro-adhesion, they are not mounted at terminal positions at which they have to be mounted, or two or more of them are mounted at any of the terminal positions. As a result, unsatisfactory bonding is caused in the package.
The coating with any of the materials disclosed in the patent documents 1 and 2 reduces the deterioration of the wetting properties at the time of melting caused by blackening and improves unsatisfactory bonding in the package caused by this deterioration. These materials are excellent in this regard. However, as a result of investigation by the present inventors, it was found that the coating with any of the lubricants and the like cannot solve the problem of the micro-adhesion of solder balls and, moreover, increases the frequency of the micro-adhesion in some cases.
The present inventors closely investigated the problem of the micro-adhesion of solder balls obtained by solidification and spheroidization in a gas phase, and consequently found that the above-mentioned micro-adhesion is markedly affected by, in particular, a humidity to which the solder balls are exposed, and is not easily caused in a dry atmosphere having a low humidity. The present inventors confirmed that the various conventional coatings described above reduce friction but accelerate adhesion in some cases. In addition, it was found that the wetting properties are deteriorated with an increase of the thickness of a coating film formed by any of these coatings.
An object of the present invention is to solve the problem of the micro-adhesion of solder balls and provide a solder ball capable of being given good wetting properties and a method for preventing the micro-adhesion of solder balls.